1. Field of the Invention
The present invention relates to an electrochromic device (hereinafter abbreviated as ECD).
2. Description of the Prior Art
ECD, because of the memory and coloring properties thereof, has been developed for various applications such as a display element, light transmission control element or memory element. Particularly ECD of the all solid-state type, or thin-film type is considered promising for practical use because of the wide variety of applications.
In general, as shown by the characteristic curve in FIG. 1, the ECD shows a decrease in the optical transmittance or transmittivity in response to the application of a voltage, and again increase in the optical transmittance to the original value upon application of an inverse voltage. The performance of the ECD is evaluated by various factors such as the optical density OD to be determined from the transmittance T.sub.o before the coloration and T after the coloration (OD=log T.sub.o /T), the magnitude of the voltage required, the coloration time t.sub.c required to reach a transmittance of 1/2, i.e. OD=0.301 after the application of the voltage, and the bleaching time t.sub.b required to undergo 90% of the change to the original state after the application of the inverse voltage at the minimum transmittance state. Thus a higher OD indicates a higher contrast, a smaller applied voltage means an easier matching with the drive circuit for the ECD, and smaller t.sub.c and t.sub.b indicate faster response of the device.
For such an all solid-state ECD with improved storage property of the coloration display, the Japanese Patent Laid-open No. 73749/1977 proposed a structure with an electro-reducible film of electrochromic material such as molybdenum oxide, tungsten oxide or niobium oxide and an electro-oxidizable film such as of chromium oxide or vanadium oxide mutually separated by an insulating film such as of titanium oxide or tantalum oxide and sandwiched between a pair of opposed electrodes. Such an ECD, under an applied voltage of 1 to 3 V, requires for undergoing a change from an initial transmittance T.sub.o of 100% to a transmittance T of 10%, i.e. for reaching an optical density OD of 1, some seconds. It is also explained in the above-mentioned patent that the bleaching required by the application of an inverse voltage takes more than 1 second and that a change in the transmittance in the range of several to ten percent occurs with the application of a square-wave pulse of .+-.10 V with a repeating period of 100 msec. Although such all solid-state ECD is superior to those previously reported, the response speed requiring approximately 1 second for the coloration and bleaching cannot be considered as acceptable for use for example in the display element or in the light control element such as the shutter or lens diaphragm in a photographic camera. In addition the change in the transmittance in the range of several to ten percent in repeated drive mode is insufficient, and it is further desirable to reduce the drive voltage, particularly the drive voltage in the repeated drive mode.